Secondary battery and method of producing the secondary battery

ABSTRACT

A secondary battery includes: an electric cell layer including a stack structure sequentially including: a positive electrode layer, a separator layer, and a negative electrode layer having an electrolyte higher in conductivity than an electrolyte of at least one of the separator layer and the positive electrode layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 12/133,733 filed on Jun. 5,2008, now U.S. Pat. No. 8,450,011, which claims the benefit of priorityfrom Japanese Patent Application No. 2007-150802 filed on Jun. 6, 2007and Japanese Patent Application No. 2008-031801 filed on Feb. 13, 2008,the entire contents of all are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a secondary battery and a method ofproducing the secondary battery, where the secondary battery is capableof bringing about high capacity and high output.

2. Description of the Related Art

A secondary battery, especially a lithium ion secondary batteryordinarily includes a positive electrode (positive electrode layer), aliquid or a solid electrolytic layer (separator layer) and a negativeelectrode (negative electrode layer). In this case, a positive electrodeactive material and a negative electrode active material are mixed witha conductive assistance, a binder and the like and then are applied to acurrent collector, to thereby form the positive electrode and thenegative electrode respectively.

The above lithium ion secondary battery is in need of higher energydensity and higher output in the trend of development, with a strategyfor thinning the secondary battery. For accomplishing the above thinsecondary battery having a light weight, one solution is a polymerbattery which is thinned by using an electrolytic part made of solid.Hereinabove, the electrolytic part was so far made of solution.

The above technology is already known in the art. However, performance(characteristic) of the recent secondary battery has been incomparablyimproved much more than when the above technology was first disclosed.

A polymer battery uses such a technology that a solid polyvinylidenefluoride (PVDF) electrolytic medium is prepared, then the thus preparedis joined with a positive electrode and a negative electrode, then aplasticizer is extracted from an entire cell prime field, then anelectrolytic solution is injected, to thereby gelate the entire cellprime field. The above gelation of the entire cell prime field caneliminate a free electrolytic solution from inside the cell. However,using the solid gelling electrolyte causes an insufficient mechanicalstrength and fails to accomplish a homogenous application of the thinfilm electrolyte, resulting in lack of practicability.

For solving the above inconveniences, U.S. Pat. No. 7,183,021 B1 {familyof Japanese Patent Application Laid-Open No. 2001-43897 (JP2001043897)}discloses a method of using a solid electrolyte in combination with aseparator.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a high-capacity and high-output secondary battery and a methodof producing the secondary battery, where the secondary batteryaccomplishes an easy charging and discharging at a great current.

According to a first aspect of the present invention there is provided asecondary battery comprising: an electric cell layer including a stackstructure sequentially including: a positive electrode layer, aseparator layer, and a negative electrode layer having an electrolytehigher in conductivity than an electrolyte of at least one of theseparator layer and the positive electrode layer.

According to a second aspect of the present invention, there is provideda method of producing a secondary battery, the method comprising thefollowing sequential operations: adhering the separator layer to each ofthe positive electrode layer and the negative electrode layer, tothereby form a stack structure of the positive electrode layer, theseparator layer and the negative electrode layer; and injecting a liquidelectrolyte to the stack structure.

According to a third aspect of the present invention, there is provideda method of producing a secondary battery, the method comprising thefollowing sequential operations: adhering the separator layer to each ofthe positive electrode layer and the negative electrode layer, tothereby form a stack structure of the positive electrode layer, theseparator layer and the negative electrode layer; injecting a liquidelectrolyte to the stack structure; and vacuum-impregnating the stackstructure.

Other objects and features of the present invention will becomeunderstood from the following description with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an electrode layer, according to first tofifth embodiments of the present invention.

FIG. 2 shows a typical lithium ion secondary battery, that is, aschematic of a cross sectional view of an entire structure of a flatstack non-bipolar lithium ion secondary battery, according to the firstembodiment of the present invention.

FIG. 3 shows a typical lithium ion secondary battery, that is, aschematic of a cross sectional view of an entire structure of a flatstack bipolar lithium ion secondary battery, according to the secondembodiment of the present invention.

FIG. 4 shows a typical lithium ion secondary battery, that is, aperspective view of a flat stack non-bipolar or bipolar lithium ionsecondary battery, according to the third embodiment of the presentinvention.

FIG. 5A, FIG. 5B and FIG. 5C show a typical pack battery, according tothe fourth embodiment of the present invention, where FIG. 5A is a planview of the pack battery, FIG. 5B is a front view of the pack batteryand FIG. 5C is a side view of the pack battery.

FIG. 6 is a schematic of a car including the pack battery, according tothe fifth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a secondary battery comprising: anelectric cell layer including a stack structure sequentially including:a positive electrode layer, a separator layer, and a negative electrodelayer having an electrolyte higher in conductivity than an electrolyteof at least one of the separator layer and the positive electrode layer.

Conventionally, a separator layer uses any of the followingelectrolytes: 1) a liquid electrolyte having such a structure that alithium salt (lithia water) as a supporting electrolyte is dissolved inan organic solvent as a plasticizer, and 2) a gel electrolyte havingsuch a structure that a liquid electrolyte is injected in a matrixpolymer including ion conductive polymer. In the former case (liquidelectrolyte), using the secondary battery for a long time deposits alithium during charging cycle of the secondary battery, causing aninternal short circuit of the secondary battery. As such, it ispreferable to use the gel electrolyte for the separator layer. However,the conventional separator, as the case may be, did not have asufficient mechanical strength, and using a solid electrolyte for all ofthe positive electrode layer, separator layer and negative electrodelayer leads to an increased resistance which may be caused by diffusionof the polymer electrolyte. Especially, using a carbon material as anegative electrode active material, as the case may be, makes itdifficult to implement charging and discharging at a great current, dueto a great interfacial resistance between the active material and thepolymer electrolyte.

Contrary to the above, according to the present invention, using a highconductivity electrolyte as a negative electrode layer having a lowerreactivity and a greater resistance than those of a positive electrodelayer can improve electrolyte transportability in the negative electrodelayer. Especially, when the polymer electrolyte is used for theseparator layer, the separator layer can be integrated with anelectrolyte layer, thus thinning the separator layer. Moreover, usingthe polymer electrolyte for the separator layer can prevent conventionalinconveniences caused in the charging cycle, namely, such inconveniencesas the above lithium deposition and the internal short circuit ofsecondary battery.

As such, the secondary battery of the present invention can bring abouthigh capacity and high output due to the following features: i) theseparator layer is thinned, ii) the electrolyte transportability in thenegative electrode layer is high, and iii) the interfacial reactivity ofthe negative electrode layer is high. The above states i), ii) and iii)can be kept. Especially, a liquid material (hereinafter otherwisereferred to as “liquid electrolyte”) used for the electrolyte of thenegative electrode layer can more effectively bring about the aboveadvantages.

At least one electric cell layer included in the secondary battery ofthe present invention has such a structure that the electrodes (i.e.,positive electrode and negative electrode) as a positive electrode layerand a negative electrode layer are opposed to each other and a separatoris interposed between the positive electrode layer and the negativeelectrode layer. In the above structure, at least one of the separatorlayer and the positive electrode layer, especially the separator layerhas an electrolyte having a conductivity lower than that of anelectrolyte of the negative electrode layer. Especially, a solidelectrolyte is interpenetrated (held) in the separator layer. With theelectrolyte interpenetrated (held) in the separator layer which is apart of a stack structure; even when the electrolyte has a contact withthe electrolytic solution in the negative electrode layer, the followinginconveniences can be suppressed or prevented: 1) solid electrolyteoutflow attributable to swelling or expansion, and 2) exfoliation ofinterface between the electrode and the separator layer, whichexfoliation is attributable to the solid electrolyte outflow. Moreover,the separator layer having a contact with each of the negative electrodelayer and the positive electrode layer can further prevent the aboveconventional inconveniencies. As such, according to the presentinvention, a liquid material having a high reactivity and an excellention conductivity can be used as an electrolyte of the positive electrodelayer or negative electrode layer, especially, the negative electrodelayer having a lower reactivity and a greater resistance than those ofthe positive electrode layer. Especially, using the polymer electrolytefor the separator layer thins the separator layer and allows thesecondary battery to bring about a still higher output.

In the following, various embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

For ease of understanding, the following description will containvarious directional terms, such as left, right, upper, lower, forward,rearward and the like. However, such terms are to be understood withrespect to only a drawing or drawings on which the corresponding part ofelement is illustrated.

Under the present invention, a secondary battery comprises: an electriccell layer including a stack structure sequentially including: apositive electrode layer, a separator layer, and a negative electrodelayer having an electrolyte higher in conductivity than an electrolyteof at least one of the separator layer and the positive electrode layer.In the above structure, it is preferable that the conductivity of theelectrolyte of the negative electrode layer is higher than theconductivity of the electrolyte of the separator layer, therebyeffectively preventing the above inconvenience which is caused by usinga polymer electrolyte for the separator layer.

In the specification, the conductivity of the electrolyte is defined bythe value calculated through the following method specified under JISKO102 where JIS stands for Japanese Industrial Standards. Specifically,a cell constant (cm⁻¹) is calculated by dividing a length (cm) (aninterval between opposing two metal plates used for measurement) by themetal plate's area (cm²).Cell constant(cm⁻¹)=Length/Area  [Expression 1]

Moreover, the thus calculated cell constant (cm⁻¹) and a resistance (Ω)which is measured otherwise are used for calculating the electrolyteconductivity (S/cm).Electrolyte conductivity=Cell constant/Resistance  [Expression 2]

More specifically, for measuring conductivity of polymer electrolyte,the following operations are taken. A polymer precursor solution isapplied to a first parting film. Then, the polymer precursor solution issandwiched by the first parting film and a second parting film, Then,the thus obtained is further sandwiched by transparent glass plates,followed by a photo polymerizing, to thereby prepare a polymer filmhaving a proper thickness. The polymer film is then sandwiched by twometal plates each fitted with a lead wire and having a certain area. Inthis state, the polymer film is subjected to measurement of itsthickness, which is defined as the length (cm). The length (cm) dividedby the metal plate's area (cm²) obtains the cell constant (cm⁻¹). Inaddition, the lead wire is connected to an impedance meter for measuringthe resistance (Ω). The cell constant (cm⁻¹) divided by the resistance(Ω) obtains the electrolyte conductivity (S/cm).

Under the present invention, it is preferable that at least one of theseparator layer and the positive electrode layer, especially theseparator layer, has a conductivity which is 1/100 to ½ relative to aconductivity of an electrolyte of the negative electrode layer. Theabove conductivity ratio less than or equal to ½ can keep a battery'spossible internal short circuit small, thereby keeping the batteryfunctional. Meanwhile, the above conductivity ratio more than or equalto 1/100 is a proper conductivity, accomplishing a sufficientperformance of an entire secondary battery to be obtained.

More preferably, at least one of the separator layer and the positiveelectrode layer has a conductivity which is 1/50 to ½ relative to theconductivity of the electrolyte of the negative electrode layer. Stillmore preferably, the above conductivity ratio is 1/20 to ½. The aboveranges of conductivity ratio do not cause the short circuit to thebattery, accomplishing a sufficient performance of the entire secondarybattery to be obtained.

Under the present invention, it is preferable that the electrolyte ofthe negative electrode layer is made of liquid material and that theelectrolyte of at least one of the separator layer and the positiveelectrode layer is made of polymer. In other words, at least one of thefollowing three structures in respective paragraphs (1), (2) and (3) ispreferred:

(1) The electrolyte of each of the negative electrode layer and thepositive electrode layer is made of liquid material while theelectrolyte of the separator layer is made of polymer.

(2) The electrolyte of each of the negative electrode layer and theseparator layer is made of liquid material while the electrolyte of thepositive electrode layer is made of polymer, and

(3) The electrolyte of the negative electrode layer is made of liquidmaterial while the electrolyte of each of the separator layer and thepositive electrode layer is made of polymer.

The above paragraphs (1) and (3) are especially preferable. Relative tothe positive electrode layer, the negative electrode layer is low inreactivity and large in resistance. Therefore, a liquid material havinga high reactivity and an excellent ion conductivity is preferable forthe electrolyte of the negative electrode layer. In the case ofparagraph (1) above, i) the separator layer has a thin film, ii) theelectrolyte transportability in the negative electrode layer is high andiii) the interfacial reactivity of the negative electrode is high. Theabove states i), ii) and iii) can be kept, to thereby obtain a batterybringing about high capacity and high output. In the case of paragraph(3) above, the electrolyte of the positive electrode layer is made ofpolymer, thereby effectively suppressing or preventing deteriorationwhich may be caused by an elution of the positive electrode. In the caseof the separator layer made of polymer electrolyte, the separator layercan be thin. Moreover, the liquid material for the electrolyte of thenegative electrode layer can bring about a high electrolytetransportability in the negative electrode layer and keep theinterfacial reactivity of the negative electrode in a high state. Assuch, the thus obtained secondary battery can bring about high capacityand high output.

<Liquid Material>

Under the present invention, the liquid material is not specificallylimited and is typically prepared by dissolving a supporting electrolytein a non-aqueous solvent. Herein, the non-aqueous solvent is notspecifically limited, examples thereof including those known in the art(plasticizer such as nonprotic solvent and the like). Examples of thenon-aqueous solvent include: cyclic carbonates such as propylenecarbonate and ethylene carbonate; chain carbonates such as dimethylcarbonate, methylethyl carbonate and diethyl carbonate; ethers such astetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxy ethane, 1,3-dioxolane and diethylether; lactonessuch as γ-butyrolactone; nitriles such as acetonitrile; esters such asmethyl propionate; amides such as dimethyl formamide; esters such asmethyl acetate and methyl formate; sulfolane; dimethyl sulfoxide;3-methyl-1,3-oxazolidine-2-on; and the like. The above non-aqueoussolvents may be used alone or in combination of two or more typesthereof. A mixture ratio in the case of combination is not specificallylimited as long as the above mixture ratio is capable of dissolving thesupporting electrolyte, and the mixture ratio may be properly selectedaccording to type of the non-aqueous solvent or according to a desiredcharacteristic.

<Supporting Electrolyte>

Moreover, the supporting electrolyte is not specifically limited andthose known (lithium salt=lithia water) may be used. Examples of thesupporting electrolyte include: inorganic acid anion salts such asLiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄, Li₂B₁₀Cl₁₀ and the like;organic acid anion salts such as LiCF₃SO₃, Li(CF₃SO₂)₂N, Li(C₂F₅SO₂)₂Nand the like; and the like. Among the above, LiPF₆ is preferably used.The above supporting electrolytes may be used alone or in combination oftwo or more types thereof.

<Polymer Electrolyte>

Under the present invention, the polymer electrolyte is not specificallylimited, examples thereof including a gel polymer electrolyte and anintrinsic (entirely solid) polymer electrolyte. Herein, the gel polymerelectrolyte is not specifically limited, examples thereof including: anion conductive solid polymer electrolyte containing an electrolyticsolution used for a conventional lithium ion secondary battery, and apolymer free of lithium ion conductivity and having a skeleton which isallowed to hold an electrolytic solution used for a conventional lithiumion secondary battery. Examples of the ion conductive solid polymerelectrolyte include a matrix polymer including ion conductive polymer,where the matrix polymer is a known solid polymer electrolyte and thelike, such as polyethylene oxide (PEO), polypropylene oxide (PPO) and acopolymer of these. The above polyalkylene oxide polymer, which iscapable of dissolving therein electrolytic salt such as lithium salt(lithia water), is preferable. Moreover, examples of the polymer free oflithium ion conductivity include polyvinylidene fluoride (PVDF),polyvinyl chloride (PVC), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) and the like, but not limited thereto. Hereinabove,PAN, PMMA and the like rather belong to a category having an ionconductivity which is small but do present. Therefore, PAN, PMMA and thelike can be so categorized as to belong to the ion conductive polymer.At least herein, however, PAN, PMMA and the like are exemplified as thepolymer used for the gel polymer electrolyte and free of lithium ionconductivity

<Gel Polymer Electrolyte>

The electrolytic solution included in the gel polymer electrolyte is notspecifically limited and is typically prepared by dissolving thesupporting electrolyte in the non-aqueous solvent, like the liquidmaterial is prepared as set forth above. In other words, the non-aqueoussolvent is not specifically limited and therefore those (plasticizersuch as nonprotic solvent and the like) set forth above can be used. Theabove non-aqueous solvents may be used alone or in combination of two ormore types thereof. A mixture ratio in the case of combination is notspecifically limited as long as the above mixture ratio is capable ofdissolving the supporting electrolyte, and the mixture ratio may beproperly selected according to type of the non-aqueous solvent or adesired characteristic. For example, when ethylene carbonate (EC) iscombined with diethyl carbonate (DEC), EC's volume relative to a totalvolume of EC and DEC is preferably 10 volume % to 80 volume %, and morepreferably 20 volume % to 60 volume %. Moreover, the supportingelectrolyte is not specifically limited and those (lithium salt=lithiawater) set forth above may be used. The above supporting electrolytesmay be used alone or in combination of two or more types thereof.Moreover, amount of the supporting electrolyte added to the non-aqueoussolvent is not specifically limited and therefore the above amount maybe the one conventionally used. A mole ratio (concentration) of thesupporting electrolyte in the non-aqueous solvent is preferably 0.5mol/dm³ to 2 mol/dm³. The above range of mole ratio can bring about asufficient reactivity (ion conductivity).

<Intrinsic (Entirely Solid) Polymer Electrolyte>

Moreover, the intrinsic (entirely solid) polymer electrolyte has such astructure that the supporting electrolyte (lithium salt=lithia water) isdissolved in the above matrix polymer and the intrinsic (entirely solid)polymer electrolyte does not include an organic solvent (non-aqueoussolvent) serving as the above plasticizer. Therefore, the intrinsicpolymer does not cause a liquid leak from the battery, thus improvingreliability of the battery. In addition, an intrinsic polymer batteryuses a solid polymer electrolyte having ion conductivity. Examples ofthe matrix polymer include a known solid polymer electrolyte such aspolyethylene oxide (PEO), polypropylene oxide (PPO) and a copolymerthereof.

Moreover, under the present invention, the above gel polymer electrolyteand the intrinsic (entirely solid) polymer electrolyte each may be usedalone or in combination of two or more types thereof. Otherwise, one ormore types of the gel polymer electrolyte(s) may be combined with one ormore types of the intrinsic (entirely solid) polymer electrolyte(s).Moreover, in the case of the single electric cell layer having theseparator layer and the positive electrode layer each including polymerelectrolyte as set forth in the above paragraph (3), such polymerelectrolytes may be same or different. For ease of production, however,the same polymer electrolytes are preferable. Likewise, in the case of asecondary battery including two or more electric cells, the electrolytesof the separator layers in the electric cells may be same or differentwhile the electrolytes of the positive electrode layers in the electriccells may be same or different. For ease of production, however, thesame polymer electrolytes are preferable.

Under the present invention, it is preferable that at least the positiveelectrode layer and the separator layer have a contact with each other.The contact between the positive electrode layer and the separator layercan prevent a possible polymer outflow which may be caused when thepolymer in a range from the surface to inner part of the separator isswelled due to an electrolytic solution oozed (exudated) out of thenegative electrode layer. It is more preferable that the separator layerhas a contact with each of the positive electrode layer and the negativeelectrode layer. The above contacts can more effectively prevent apossible polymer outflow which may be caused when the polymer in a rangefrom the surface to inner part of the separator is swelled due to anelectrolytic solution oozed (exudated) out of the negative electrodelayer or positive electrode layer.

In this specification, the language “the positive electrode layer havinga contact with the separator layer” signifies a state which is free ofelectrolyte outflow even when a liquid material in the negativeelectrode layer is exudated (oozed) out, where the electrolyte outflowmay be caused by a swelling of the electrolyte (especially, polymerelectrolyte) in a range from the surface to inner part of the separator.Meanwhile, the language “the separator layer having a contact with eachof the positive electrode layer and the negative electrode layer”signifies a like state. Specifically, in the case of “the positiveelectrode layer having a contact with the separator layer,” the contactstate between the positive electrode layer and the separator layerincludes the followings: 1) a first state where a stack structure of thepositive electrode layer and separator layer is pressed (pressurewelding) in a direction of thickness of the stack structure with apressure, and 2) a second state where the positive electrode layer isadhered to the separator layer by polymerizing the followings in thepresence of a polymerizing initiator: i) an adhesive, and ii) anelectrolyte in the separator layer. Of the above first and secondstates, it is preferable to adhere the positive electrode layer to theseparator layer. It is more preferable to adhere the separator layer toeach of the positive electrode layer and the negative electrode layer.

<Groove of Positive Electrode Layer or Negative Electrode Layer>

Under the present invention, in the case of the electrode layer(positive electrode layer and negative electrode layer) including acurrent collector, and an active material layer formed on the currentcollector, it is preferable that at least one of the positive electrodeactive material layer and the negative electrode active material layeris so configured to have a groove, preferably, on a side contacting thecurrent collector.

For impregnating the electrolyte in the production of the electrodelayer, the above grooved structure can improve electrolytic permeability(of an after-described polymer electrolytic precursor and anafter-descried liquid electrolyte) to the electrode layer's central partinto which the liquid is unlikely to permeate. The polymer electrolyticprecursor has an electrolyte which is less permeable than the liquidelectrolyte. Therefore, it is preferable that the groove is formed atleast in the electrode layer for impregnating the polymer electrolyticprecursor. The active material layer and the current collector are to beset forth afterward.

<Shape and Size of Groove of Positive Electrode Layer or NegativeElectrode Layer>

The shape and size of the groove of the positive electrode layer ornegative electrode layer are not specifically limited. The groove mayhave an arbitrary cross section as long as the electrolytic solution caninterpenetrate in the groove, examples of the groove shape includingsquare, rectangle, quadrangle, equilateral triangle, isosceles triangle,triangle, semicircle, semiellipse and the like. The groove may have anarbitrary size as long as the electrolytic solution is likely tointerpenetrate in the positive electrode layer or negative electrodelayer. Preferably, the groove has a width 100% to 5000% of an averageparticle diameter of the positive electrode active material (forpositive electrode layer) or negative electrode active material (fornegative electrode layer). Moreover, the groove has a volume of 5% to30% of a volume of the positive electrode layer or negative electrodelayer, where the above percentage is calculated by the followingequation: (Total volume of groove/Volume of electrode layer)×100(%). Thegroove having the above shape and size allows the electrolyte (liquidelectrolyte, polymer electrolytic precursor and the like) tosufficiently permeate in the electrode layer's central part into whichthe liquid is unlikely to permeate.

<Direction of Groove>

Moreover, the direction for forming the groove is not specificallylimited as long as the electrolyte can conveniently permeate in thepositive electrode layer or negative electrode layer. Examples of thegroove direction include i) longitudinal and lateral in a form of agrid, ii) parallel in a constant direction, iii) honeycomb (hexagonal),and the like. Of the above, parallel in a constant direction,especially, the direction for injecting the electrolytic solution ispreferable, so that the electrolytic solution can be smoothly injectedinto the groove.

<Method of Forming Groove of Positive Electrode Layer or NegativeElectrode Layer>

The method for forming the groove of positive electrode layer ornegative electrode layer is not specifically limited, examples thereofincluding the following: 1) to a surface of the current collector,patterning (as a first sublayer) a slurry including an active material,so as to form a groove, and 2) transcribing a second sublayer on thefirst sublayer, so as to form the second sublayer. The above patterningmay be those known in the art, examples thereof including spray coat,screen print, ink jet and the like. FIG. 1 shows a schematic of anelectrode layer. In FIG. 1, an electrode layer 530 includes a currentcollector 500, and an active material layer 510 on the current collector500. Patterning can form a first active material sublayer 511. In thiscase, the patterning is so implemented that a groove 520 is formed aftera second active material sublayer 512 is applied. Then, the secondactive material sublayer 512 substantially equivalent to an entire areaof the collect collector 500 is transcribed on the first active materialsublayer 511, thereby forming the electrolyte layer 530 having thegroove 520.

<Method of Producing Secondary Battery>

The method for producing the secondary battery of the present inventionis not specifically limited, examples of the production method includingthose known in the art or a properly modified version thereof.Hereinafter described is a method of producing an electric cell layerhaving such a structure that i) the electrolyte of each of the positiveelectrode layer and the negative electrode layer is liquid, ii) theelectrolyte of the separator is polymer, and iii) the separator layerhas a contact with each of the positive electrode layer and the negativeelectrode layer. Note that the method for adhering the positiveelectrode layer to the separator layer is to be omitted, since thismethod only excludes the negative electrode layer.

Specifically, the method for producing the secondary battery includesthe following operations.

Operation (1) Dissolving a supporting electrolyte in a non-aqueoussolvent, to thereby prepare an electrolytic solution.

Operation (2) adding the above-described matrix polymer, supportingelectrolyte and polymerizing initiator to the thus prepared electrolyticsolution, to thereby prepare an electrolytic precursor solution.

Operation (3) Dipping a separator substrate in the above electrolyticprecursor solution, followed by removing of an excessive electrolyticprecursor solution, to thereby prepare an impregnated separator.

Operation (4) Sandwiching the thus impregnated separator between apositive electrode layer and a negative electrode layer, followed bypolymerizing of the electrolyte of the impregnated separator, to therebyadhere the separator layer to each of the positive electrode layer andthe negative electrode layer.<Operation (1)>

In the operation (1), the non-aqueous solvent is not specificallylimited and therefore those known in the art (plasticizer such asnonprotic solvent and the like) are available. Examples of thenon-aqueous solvent include: cyclic carbonates such as propylenecarbonate and ethylene carbonate; chain carbonates such as dimethylcarbonate, methylethyl carbonate and diethyl carbonate; ethers such astetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxy ethane, 1,3-dioxolane and diethylether; lactonessuch as γ-butyrolactone; nitriles such as acetonitrile; esters such asmethyl propionate; amides such as dimethyl formamide; esters such asmethyl acetate and methyl formate; sulfolane; dimethyl sulfoxide;3-methyl-1,3-oxazolidine-2-on; and the like. The above non-aqueoussolvents may be used alone or in combination of two or more typesthereof. A mixture ratio in the case of combination is not specificallylimited as long as such the above mixture ratio is capable of dissolvingthe supporting electrolyte, and the mixture ratio may be properlyselected according to type of the non-aqueous solvent or according to adesired characteristic. For example, when ethylene carbonate (EC) iscombined with diethyl carbonate (DEC), EC's volume relative to a totalvolume of the EC and DEC is preferably 10 volume % to 80 volume %, andmore preferably 20 volume % to 60 volume %.

In the operation (1), the supporting electrolyte is not specificallylimited and those known in the art (lithium salt=lithia water) may beused. Examples of the supporting electrolyte include: inorganic acidanion salts such as LiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄,Li₂B₁₀Cl₁₀ and the like; organic acid anion salts such as LiCF₃SO₃,Li(CF₃SO₂)₂N, Li(C₂F₅SO₂)₂N and the like; and the like. The abovesupporting electrolytes may be used alone or in combination of two ormore types thereof. Moreover, amount of the supporting electrolyte addedto the non-aqueous solvent is not specifically limited and therefore theabove amount may be the one conventionally used. A mole ratio(concentration) of the supporting electrolyte in the non-aqueous solventis preferably 0.5 mol/dm³ to 2 mol/dm³. The above range of mole ratiocan bring about a sufficient reactivity (ion conductivity).

<Operation (2)>

In the operation (2), the matrix polymer, supporting electrolyte andpolymerizing initiator are added to the electrolytic solution preparedin the operation (1), to thereby prepare the electrolytic precursorsolution. Herein, the matrix polymer is preferably the polymerelectrolyte as set forth above. Polyethylene oxide (PEO), polypropyleneoxide (PPO) and a copolymer thereof are more preferable. Thepolyethylene oxide (PEO) is especially preferable. In this case, theelectrolyte of the separator layer may be used alone or in combinationof two or more types thereof. Moreover, the electrolyte of the separatorlayer may be the same as or different from an ion conductive polymerused for at least one of the positive electrode layer and negativeelectrode layer of the secondary battery under the present invention, tobe described afterward. Being the same is more preferable.

In the operation (2), the polymerizing initiator is added for acting ona cross-linking base of the matrix polymer (polymer electrolyte), so asto progress cross-liking reaction. According to external factors forallowing the polymerizing initiator to act as an initiator, thepolymerizing initiator is categorized into a photo polymerizinginitiator, a thermal polymerizing initiator and the like. Examples ofthe polymerizing initiator include azobisisobutyronitrile (AIBN) as thethermal polymerizing initiator, benzyl dimethyl ketal (BDK) as the photopolymerizing initiator, and the like. Preferably, theazobisisobutyronitrile (AIBN) as the thermal polymerizing initiator isused. Amount of the polymerizing initiator added to the electrolyticsolution is not specifically limited. The polymerizing initiator ispreferably added by 100 mass ppm to 10,000 mass ppm relative to thematrix polymer, and more preferably 100 mass ppm to 1,000 mass ppm.

<Operation (3)>

Then, in the operation (3), the separator substrate is dipped in theelectrolytic precursor solution prepared in the operation (2). In thiscase, the separator substrate is not specifically limited and thereforethose known in the art may be used. Examples of the separator substrateinclude: polyolefin resins such as fine-pore polyethylene film,fine-pore polypropylene film and fine-pore ethylene-propylene polymerfilm, porous film or nonwoven fabric of component(s) such as aramid,polyimide and cellulose, a stack structure of component(s) such asaramid, polyimide and cellulose, and the like. The above can bring aboutan excellent effect of suppressing the separator substrate's reactivitywith the electrolyte (electrolytic solution). Other examples of theseparator substrate include a complex resin film which is made by usingpolyolefin resin nonwoven fabric or polyolefin resin porous film as areinforcing material layer and filling vinylidene fluoride resincompound in the reinforced material layer.

Thickness of the separator substrate may be properly determinedaccording to application. For a secondary battery for driving a motor ofan automobile and the like, the separator substrate preferably has athickness of 1 μm to 100 μm. Moreover, porosity, size and the like ofthe separator substrate may be properly determined in view ofcharacteristics of the prepared secondary battery. For example, voidage(porosity) of the separator substrate is preferably 30% to 80%, and morepreferably 40% to 70%. The separator substrate having voidage of 40% to70% can bring about a secondary battery causing a higher output.Curvature of the separator layer is preferably 1.2 to 2.8. The separatorsubstrate having the above voidage (porosity) can sufficiently introducethe electrolytic solution and the separator layer's electrolyte, and cansufficiently keep a strength of the separator layer.

Moreover, conditions for dipping the separator substrate in theelectrolytic precursor solution is not specifically limited as long asthe electrolytic precursor solution sufficiently interpenetrates in theseparator substrate. Specifically, the following dipping conditions arepreferable: 15° C. to 60° C., and more preferably 20° C. to 50° C.; 1min to 120 min, and more preferably 5 min to 60 min. After the dippingunder a certain condition, an excessive electrolytic precursor solutionis to be removed. The removing method is not specifically limited andtherefore those known in the art may be used. For example, the followingremoving methods are preferable:

First method: i) Between parting films, sandwiching the separatorsubstrate including the interpenetrated electrolytic precursor solution,and

-   -   ii) slightly brandishing the separator substrate with rolling        and the like.

Second method: Slightly squeezing the separator substrate including theinterpenetrated electrolytic precursor solution.

<Operation (4)>

Moreover, in the operation (4), the separator impregnated in theoperation (3) is sandwiched by the positive electrode layer and thenegative electrode layer, followed by polymerizing of the electrolyte ofthe impregnated separator, to thereby adhere the separator layer to eachof the positive electrode layer and the negative electrode layer. Withthe impregnated separator thus sandwiched by the positive electrodelayer and the negative electrode layer, a part of the electrolyticprecursor solution in the impregnated separator moves to interfacesbetween the positive electrode layer and the separator layer and betweenthe negative electrode layer and the separator layer. Polymerizing inthis state is considered to bring about the following effect. Theelectrolytes (the matrix polymer of gel electrolyte and the matrixpolymer of intrinsic polymer electrolyte) in the separator layer and inthe above interfaces mutually form a cross-liking structure foradhesion, thus bringing about an excellent mechanical strength. Theabove adhesion mechanism is merely based on a supposition and is notlimited thereto. Moreover, the polymerizing method is not specificallylimited as long as the above cross-linking structure is formed. Examplesof the polymerizing method include thermal polymerizing, photopolymerizing (especially, ultraviolet polymerizing, radiationpolymerizing and electron beam polymerizing) and the like to beimplemented on the electrolyte {such as polyethylene oxide (PEO) andpolyphenylene oxide (PPO)} of the separator layer. The thermalpolymerizing is more preferable.

In the operation (4), after the sandwiching of the impregnated separatorby the positive electrode layer and negative electrode layer and beforethe polymerizing of the electrolyte of the separator layer, it ispreferable that the stack structure of the positive electrode layer,impregnated separator and negative electrode layer is fixed with twoplates (for example, glass, parting film and the like). Moreover, it ispreferable that the polymerizing reaction is implemented in a laminatebag and the like. As such, the stack structure's deviation in thedirection along the face can be prevented and the film thicknessvariation of the stack structure in the polymerizing can be prevented.

In the operation (4), polymerizing conditions are not specificallylimited as long as the separator layer can have a sufficient contactwith each of the positive electrode layer and the negative electrodelayer. In the case of the thermal polymerizing, for example, heating thestack structure of the positive electrode layer, impregnated separatorand negative electrode layer in the following conditions is preferable:20° C. to 150° C., more preferably 30° C. to 100° C.; 10 min to 10 hr,more preferably 30 min to 5 hr.

In the operation (4), the positive electrode layer and the negativeelectrode layer can be formed in a known method, except that thepositive and negative electrode layers are each free from electrolyte.Specifically, the positive electrode layer and the negative electrodelayer respectively may include positive electrode active material andnegative electrode active material, or if necessary, may includeelectrolytic salt for enhancing ion conductivity, conductivity assistantfor enhancing electron conductivity, binder and the like.

<Positive Electrode Active Material>

Herein, the positive electrode active material has such a composition asto occlude ions during discharging while discharge ions during charging.One preferable example of the positive electrode active material is acomposite oxide including transition metal and lithium. Examples of thepositive electrode active material include: Li—Co composite oxide suchas LiCoO₂, Li—Ni composite oxide such as LiNiO₂, Li—Mn composite oxidesuch as spinel LiMn₂O₄, Li—Fe composite oxide such as LiFeO₂, and theone having a part of the above transition metals which part issubstituted with another element. The above lithium-transition metalcomposite oxides are materials excellent in reactivity as well as cyclicdurability and low in cost. Therefore, using the abovelithium-transition metal composite oxides for the electrode works forforming a battery having an excellent output characteristic. Otherexamples of the positive electrode active material include: phosphoricacid compound (including transition metal and lithium) such as LiFePO₄,and sulfuric acid compound; transition metal oxide or transition metalsulfide such as V₂O₅, MnO₂, TiS₂, MoS₂ and MoO₃; PbO₂, AgO, NiOOH; andthe like. The above positive electrode active materials may be usedalone or in combination of two or more types thereof. An averageparticle diameter of the positive electrode active material is notspecifically limited, preferably 1 μm to 100 μm and more preferably 1 μmto 20 μm in view of the positive electrode active material's highercapacity, reactivity and cyclic durability. Within the above range, thesecondary battery can be prevented from having an increased internalresistance which may be caused during charging and discharging under ahigh output condition, thus taking out a sufficient current. In the caseof the positive electrode active material being a secondary particle, itis preferable that a primary particle which is a constituent of thesecondary particle has an average particle diameter of 10 nm to 1 μm,but not necessarily limited thereto under the present invention.Depending on the production method, however, it is not necessary thatthe positive electrode active material is made into the secondaryparticle by aggregation or agglomeration. The particle diameter of thepositive electrode active material and the diameter of the primaryparticle each are a median obtained by a laser diffraction method. Shapeof the positive electrode active material varies with type, productionmethod and the like, examples thereof including sphere (powder), plate,needle, column, edge and the like, but not limited thereto. Preferably,an optimum shape of the positive electrode active material is selectedso as to accomplish improvement of the battery performance such ascharging and discharging.

<Negative Electrode Active Material>

Moreover, the negative electrode active material has such a compositionas to discharge ions during discharging while occlude ions duringcharging. Examples of the negative electrode active material include:metals such as Si and Sn, metal oxides such, as TiO, Ti₂O₃ and TiO₂ orSiO₂, SiO and SnO₂, composite oxides (including lithium and transitionmetal) such as Li_(4/3)Ti_(5/3)O₄ and Li₇MnN, Li—Pb alloy, Li—Al alloy,Li, and carbon materials such as natural graphite, artificial graphite,carbon black, active carbon, carbon fiber, coke, soft carbon and hardcarbon. The above negative electrode active materials may be used aloneor in combination of two or more types thereof. An average particlediameter of the negative electrode active material is not specificallylimited, preferably 1 μm to 100 μm and more preferably 1 μm to 20 μm inview of the negative electrode active material's higher capacity,reactivity and cyclic durability. Within the above range, the secondarybattery can be prevented from having an increased internal resistancewhich may be caused during charging and discharging under a high outputcondition, thus taking out a sufficient current. In the case of thenegative electrode active material being a secondary particle, it ispreferable that a primary particle which is a constituent of thesecondary particle has an average particle diameter of 10 nm to 1 μm,but not necessarily limited thereto under the present invention.Depending on the production method, however, it is not necessary thatthe negative electrode active material is made into the secondaryparticle by aggregation or agglomeration. The particle diameter of thenegative electrode active material and the diameter of the primaryparticle each are a median obtained by a laser diffraction method. Shapeof the negative electrode active material varies with type, productionmethod and the like, examples thereof including sphere (powder), plate,needle, column, edge and the like, but not limited thereto. Preferably,an optimum shape of the negative electrode active material is selectedso as to accomplish improvement of the battery performance such ascharging and discharging.

<Electrolytic Salt>

The electrolytic salt is not specifically limited, examples thereofincluding: BETI {lithiumbis, (perfluoroethylene sulfonylimide);otherwise denoted by Li(C₂F₅SO₂)₂N}, LiBF₄, LiPF₆, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, LiBOB (lithiumbis oxide borate) and an arbitrarycombination thereof.

<Conductivity Assistant>

Examples of the conductivity assistant include acetylene black, carbonblack, ketjen black, vapor grown carbon fiber, carbon nanotube, expandedgraphite, graphite and the like, but not specifically limited thereto.

<Binder>

Examples of the binder include polyvinylidene fluoride (PVDF), styrenebutadiene rubber (SBR), polyimide, polytetrafluoroethylene (PTFE) andthe like, but not specifically limited thereto.

<Positive Electrode Layer, Negative Electrode Layer and CurrentCollector>

The positive electrode layer and the negative electrode layer each havesuch a structure that the active material layer including theelectrolyte, electrolytic salt, conductive assistant and binder isordinarily formed on a proper current collector. The material for thecurrent collector is not specifically limited, examples thereofincluding at least one selected from the group consisting of iron,chromium, nickel, manganese, titanium, molybdenum, vanadium, niobium,aluminum, copper, silver, gold, platinum and carbon, more preferably atleast one selected from the group consisting of aluminum, titanium,copper, nickel, silver and stainless (SUS). The above materials for thecurrent collector may have a single-layer structure (e.g. foil) or amulti-layer structure including different types of materials. Otherwise,a clad material coated with the above materials (e.g. a clad materialincluding nickel and aluminum or a clad material including copper andaluminum) may be used. Moreover, a plating material which is acombination of the above current collector materials may be preferablyused. Furthermore, a surface of a metal (other than aluminum) which isany of the above current collector materials may be coated with aluminumwhich is another current collector material. As the case may be, metalfoils which are two or more of the above current collector materials maybe attached and mated for forming the current collector to be used. Theabove materials are excellent in corrosion resistance, conductivity,machinability and the like. The current collector has a typicalthickness of 5 μm to 50 μm but not specifically limited thereto.

Size of the current collector is determined according to the applicationof the battery. The current collector has a large area for preparing alarge electrode for a large battery while having a small area forpreparing a small electrode for a small battery.

A method of forming the positive electrode (negative electrode as well)on the surface of the current collector is not specifically limited andtherefore those known in the art may be likewise used. For example, asset forth above, the positive electrode active material (or the negativeelectrode active material), or if necessary, the electrolytic salt forenhancing ion conductivity, conductive assistant for enhancing electronconductivity, and binder is/are dispersed or dissolved in a propersolvent, to thereby prepare a positive electrode active materialsolution (or a negative electrode active material solution). Then, thethus prepared positive electrode active material solution (negativeelectrode active material solution) is applied to the current collector,followed by drying for removing the solvent, and still followed bypressing, to thereby form the positive electrode layer (or negativeelectrode layer) on the current collector. In the above operations, thesolvent is not specifically limited, examples thereof includingN-methyl-2-pyrrolidone (NMP), dimethyl formamide, dimethyl acetoamide,methyl formamide, cyclohexane, hexane and the like. For polyvinylidenefluoride (PVDF) as the binder, NMP is preferable as the solvent.

In the above method, after the positive electrode active materialsolution (or negative electrode active material solution) is applied tothe current collector and followed by drying, pressing is implemented.Adjusting the pressing conditions can control the voidage of thepositive electrode layer (or negative electrode layer).

Measures or conditions for the pressing are not specifically limited,and therefore can be properly adjusted such that the positive electrodelayer (or negative electrode layer) has a proper voidage after thepressing. Examples of measures for the pressing include hot press,calendar roll press and the like. Moreover, the pressing conditions(temperature, pressure and the like) are not specifically limited andtherefore those conventionally known may be used.

Thickness of each of the positive electrode layer and the negativeelectrode layer is not specifically limited, preferable examples thereofincluding 10 μm to 200 μm, especially 20 μm to 100 μm. In this case, thepositive electrode layer and the negative electrode layer may have thesame thickness or different thicknesses.

Moreover, each of the positive electrode layer and the negativeelectrode layer may have a single-layer structure or a multi-layer stackstructure. The number of layers for the multi-layer stack structure isnot specifically limited, a preferable example thereof being 1 to 3 inview of liquid electrolyte's injecting property (how easy to inject) andion conductivity which are to be described afterward.

In the case of the positive electrode layer and the negative electrodelayer each having a multi-layer stack structure, it is preferable thatsuch multiple layers have different voidages in the thickness directionof the negative electrode layer. Herein, at least one of the positiveelectrode layer and the negative electrode layer has the abovemultiple-layer stack structure having different voidages. Preferably,however, at least the negative electrode layer has the above structure.Moreover, the above multi-layer stack structure can be accomplished byrepeatedly forming the positive electrode layers (negative electrodelayers as well) through the above method.

Typically, for injecting the liquid electrolyte in the direction alongthe surface of the stack structure, the liquid electrolyte is mostunlikely to be permeated around a center area of each layer. Therefore,injecting the liquid electrolyte in each step may cause a portion wherethe liquid electrolyte is not permeated. Therefore, as set forth above,at least one coarse layer (i.e., grooved layer) present in the directionalong the surface can progress the liquid electrolyte to permeate in thedirection along the face. The direction of surface depth has a shortdistance for the liquid electrolyte to permeate. Therefore, in thedirection of surface depth, the liquid electrolyte can sufficientlypermeate and secure a sufficient ion conductivity. Herein, in the caseof the positive electrode layer and the negative electrode layer eachhaving a single-layer structure, such single layer preferably has avoidage of 30% to 60%. Likewise, in the case of the positive electrodelayer and the negative electrode layer each having a multi-layer stackstructure, each of the multiple layers preferably has a voidage of 30%to 60%. Each layer having a voidage of 30% or more can secure asufficient void, providing a sufficient amount of liquid materialpermeated. On the contrary, each layer having a voidage of 60% or lesscan secure a sufficient capacity for the secondary battery. Moreover,for impregnating the polymer electrolytic precursor in the electrodelayer, the voidage of 30% to 60% can likewise progress permeation of theelectrolyte.

The above operations (1) to (4) can adhere the separator layer to eachof the positive electrode layer and the negative electrode layer. Then,a liquid material (liquid electrolyte) is injected to the thus obtainedstack structure of the positive electrode layer, separator layer andnegative electrode layer (injecting operation). Specifically, thepreferable method of producing the secondary battery under the presentinvention includes the following operations: adhering the separatorlayer to each of the positive electrode layer and the negative electrodelayer, to thereby prepare a stack structure of the positive electrodelayer, separator layer and negative electrode layer, then, injecting theliquid electrolyte to the stack structure. The above operations (1) to(4) in combination with the injecting operation can form the stackstructure of the positive electrode layer, separator layer and negativeelectrode layer, where each of the positive electrode layer and thenegative electrode layer has an electrolyte which is a liquid material(liquid electrolyte) while the separator layer has a polymerelectrolyte. Otherwise, as set forth above under the present invention,it is not necessary that both of the negative electrode layer and thepositive electrode layer have the electrolyte which is a liquidmaterial. For example, in the case of the negative electrode layerhaving an electrolyte which is a liquid material and the positiveelectrode layer having an electrolyte which is a polymer electrolyte,the secondary battery can be produced in the following operations:First, the positive electrode layer produced in the above method isdipped in the electrolytic precursor solution prepared in the operation(2), to thereby form a positive electrode layer including electrolyte.Then, like the operation (4), the impregnated separator prepared in theoperation (3) is sandwiched by the positive electrode layer and thenegative electrode layer, followed by polymerizing of the electrolyte ofthe impregnated separator, to thereafter adhere the separator layer toeach of the positive electrode layer and the negative electrode layer.Otherwise, the operation (4) may be repeated, except for using thefollowing positive electrode layer: A positive electrode layer which isprepared in a manner like a known method, that is, in such a state as toinclude an electrolyte.

In the above injecting operations, the injecting method is notspecifically limited as long as the liquid electrolyte is sufficientlypermeated in the positive electrode layer and the negative electrodelayer each not including electrolyte. Specifically, the followinginjecting methods are preferable:

Method 1:

-   -   putting the stack structure of the positive electrode layer,        separator layer and negative electrode layer into a laminate        bag, and    -   injecting the electrolytic solution to the laminate bag.        Method 2 (vacuum impregnating operation):    -   putting the stack structure of the positive electrode layer,        separator layer and negative electrode layer into a laminate        bag,    -   injecting the electrolytic solution to the laminate bag, and    -   packing the laminate in a vacuum state.

Herein, the stack structure of the positive electrode layer, separatorlayer and negative electrode layer has the separator layer adhered toeach of the positive electrode layer and the negative electrode layer.Therefore, the method 2 (vacuum impregnating operation) is preferable.In the method 2, the electrolytic solution can permeate to such anextent as to reach the layers' adhesion parts or the electrode layer'scentral part, where the electrolytic solution is unlikely to permeate inthe central part.

In sum, the more preferable method of producing the secondary batteryunder the present invention includes the following sequentialoperations: adhering the separator layer to each of the positiveelectrode layer and the negative electrode layer, to thereby prepare thestack structure of the positive electrode layer, separator layer andnegative electrode layer, injecting the liquid electrolyte into thestack structure, and vacuum impregnation.

In the above injecting operation, it is preferable that a pressure isapplied in a direction perpendicular to the surface of the stackstructure (i.e., thickness direction of stack structure), so as not tochange (or causing a small variation is allowed) thickness of the stackstructure of the positive electrode layer, separator layer and negativeelectrode layer. The above applying of the pressure can be accomplished,for example, by sandwiching the stack structure by two glass plates. Inthe operation (4), the stack structure sandwiched by two glass plates isput into the laminate bag during the thermal polymerizing operation. Itis preferable that the stack structure in this state is subjected to theinjecting operation. Moreover, in the injecting operation, the pressureto be applied in the direction perpendicular to the surface of the stackstructure (i.e., thickness direction of stack structure) is notspecifically limited as long as such a pressure does not change (orcausing a small variation is allowed) thickness of the stack structureduring the injecting operation. For example, the following pressingoperations are preferably used: securing two glass plates with a clipand the like so as to keep thickness of the stack structure, andpressing the stack structure with a spring and the like. Preferably, thethickness of the stack structure in the injecting operation is soadjusted as to have a variation less than or equal to 5% of a certainthickness (distance between two glass plates when the stack structure issandwiched therebetween), and more preferably 0.01% to 1% of the certainthickness.

<Groove of Current Collector>

Moreover, under the present invention, when the current collector usedin the forming of the positive electrode layer and negative electrodelayer is flat like a foil, it is preferable to form a groove on thesurface of the current collector so as to improve the electrolyte(polymer electrolytic precursor or liquid electrolyte)'s permeability tothe central part. It is preferable that the positive electrode layerincluding the positive electrode active material is formed on thecurrent collector having the groove which has width and depth each lessthan or equal to 10% of an average particle diameter of the positiveelectrode active material. It is likewise preferable that the negativeelectrode layer including the negative electrode active material isformed on the current collector having the groove which has width anddepth each less than or equal to 10% of an average particle diameter ofthe negative electrode active material. Shape and size of the groove arenot specifically limited. For example, cross section of the groove isnot specifically limited as long as the electrolytic solution easilyinterpenetrates in the current collector, examples of the cross sectionof the groove shape including square, rectangle, quadrangle, equilateraltriangle, isosceles triangle, triangle, semicircle, semiellipse and thelike. The groove may have an arbitrary size as long as the electrolyticsolution is likely to interpenetrate in the current collector.Preferably, the width of the groove is less than or equal to 10% of theaverage particle diameter of each of the positive electrode activematerial (in the case of positive electrode layer) and the negativeelectrode active material (in the case of negative electrode layer).Preferably, the depth of the groove is less than or equal to 10% of theaverage particle diameter of each of the positive electrode activematerial (in the case of positive electrode layer) and the negativeelectrode active material (in the case of negative electrode layer).Moreover, the groove's volume relative to the current collector's volume{(groove's total volume/current collector's volume)×100(%)} ispreferably 1 volume % to 30 volume %. The groove having the above shapeand size allows the electrolyte to sufficiently permeate to theelectrode layer's central part into which the electrolyte is unlikely topermeate. Moreover, the direction for forming the groove is notspecifically limited as long as such a direction allows the electrolyticinterpenetration. Examples of the direction of groove include i)longitudinal and lateral in a form of a grid, ii) parallel in a constantdirection, iii) honeycomb (hexagonal) and the like. Of the above,parallel in a constant direction, especially, the direction forinjecting the electrolyte is preferable, thereby the electrolyte can besmoothly injected into the groove.

Moreover, the liquid electrolyte used for the above injecting operationis not specifically limited, for example, the electrolytic solution inthe above operation (1) may be likewise used. In other words, the liquidelectrolyte is prepared by dissolving the supporting electrolyte in thenon-aqueous solvent. Examples of the non-aqueous solvent usable for theabove injecting operation include: cyclic carbonates such as propylenecarbonate and ethylene carbonate; chain carbonates such as dimethylcarbonate, methylethyl carbonate and diethyl carbonate; ethers such astetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxy ethane, 1,3-dioxolane and diethylether; lactonessuch as γ-butyrolactone; nitriles such as acetonitrile; esters such asmethyl propionate, amides such as dimethyl formamide; esters such asmethyl acetate and methyl formate; sulfolane; dimethyl sulfoxide;3-methyl-1,3-oxazolidine-2-on; and the like. Preferably, ethylenecarbonate and diethyl carbonate are used. The above non-aqueous solventsmay be used alone or in combination of two or more types thereof. Amixture ratio in the case of combination is not specifically limited aslong as the above mixture is capable of dissolving the supportingelectrolyte, and the mixture ratio may be properly selected according totype of the non-aqueous solvent or according to a desiredcharacteristic. For example, when ethylene carbonate (EC) is combinedwith diethyl carbonate (DEC), EC's volume relative to a total volume ofEC and DEC is preferably 10 volume % to 80 volume %, and more preferably20 volume % to 60 volume %.

Moreover, the supporting electrolyte is not specifically limited andthose known in the art (lithium salt=lithia water) may be used. Examplesof the supporting electrolyte include: inorganic acid anion salts suchas LiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄, Li₂B₁₀Cl₁₀ and thelike; organic acid anion salts such as LiCF₃SO₃, Li(CF₃SO₂)₂N,Li(C₂F₅SO₂)₂N and the like; and the like. The above supportingelectrolytes may be used alone or in combination of two or more typesthereof. Moreover, amount of the supporting electrolyte added to thenon-aqueous solvent is not specifically limited and therefore the aboveamount may be the one conventionally used. A mole ratio (concentration)of the supporting electrolyte in the non-aqueous solvent is preferably0.5 mol/dm³ to 2 mol/dm³. The above range of mole ratio can bring abouta sufficient reactivity (ion conductivity).

The secondary battery of the present invention includes at least one ofthe electric cells prepared in the above manner. The electric cell ofthe present invention is received in a cell case and the like. The cellcase is not specifically limited as long as such a cell case isresistant to an external shock or an environmental deterioration whenthe cell is used. For example, a cell case made of laminate materialshaving a compound stack of polymer films and metal foils can be used,where a periphery of the cell case is joined through heat sealing. Inaddition, another cell case has such a structure that an open partthereof (when such another cell case is in a form of a bag) is heatsealed, and a positive electrode lead terminal and a negative electrodelead terminal are lead out of the thus heat sealed part. The number ofparts for taking out the respective positive and negative lead terminalsis not limited to one. Moreover, the materials for the cell case are notspecifically limited to the above, other examples thereof includingplastic, metal, rubber and the like, or an arbitrary combinationthereof. Shape of the cell case is not specifically limited, examplesthereof including film, plate, box and the like. Moreover, providing aterminal for conducting an inside with an outside of the cell case isallowed. In this structure, for taking out the current, the currentcollector is connected to the inside of the terminal while the leadterminal is connected to the outside of the terminal.

<Structure of Secondary Battery>

Configuration or structure of the secondary battery of the presentinvention is not specifically limited, examples thereof including stack(flat), rolled (cylindrical) and the like which are conventionally knownin the art. Moreover, in view of electrical connection (electrodestructure) in the lithium ion secondary battery, the secondary batteryof the present invention may have an internal parallel connection or aninternal serial connection.

Under the present invention, adopting the stack (flat) battery structurecan secure a long-term reliability due to a sealing technology such asan easy thermocompression bonding, which is advantageous in terms ofcost and workability (operatability).

Referring to drawings, a lithium ion secondary battery having aninternal parallel connection and a lithium ion secondary battery havingan internal serial connection under the present invention are to be setforth. The present invention is, however, not limited thereto.

First Embodiment

FIG. 2 shows a typical lithium ion secondary battery, according to afirst embodiment of the present invention. More specifically, FIG. 2shows a schematic of a cross sectional view of an entire structure of aflat (stack) non-bipolar lithium ion secondary battery (hereinafter,otherwise referred to as “non-bipolar lithium ion secondary battery” or“non-bipolar secondary battery” for short).

As shown in FIG. 2, a non-bipolar lithium ion secondary battery 10according to the first embodiment has a cell outer package 22 using acomposite laminate film including polymer and metal. Joining entirety ofthe periphery of the laminate film through heat sealing brings aboutsuch a structure that a generating element (cell element) 17 is sealedand received in the cell outer package 22. Herein, the generatingelement 17 has such a structure that a positive electrode plate, aseparator layer 13 and a negative electrode plate are stacked, where thepositive electrode plate has a positive electrode current collector 11having first and second faces each formed with a positive electrode(positive electrode active material layer) 12 while the negativeelectrode plate has a negative electrode current collector 14 havingfirst and second faces each formed with a negative electrode (negativeelectrode active material layer) 15. In the above structure, via theseparator layer 13, the positive electrode (positive electrode activematerial layer) 12 on the first face of the first positive electrodeplate faces the negative electrode (negative electrode active materiallayer) 15 on the first face of the first negative electrode plateadjacent to the first positive electrode plate, thus forming a pluralityof the positive electrode plates, separator layers 13 and negativeelectrode plates which are stacked.

With the above structure, the positive electrode (positive electrodeactive material layer) 12, separator layer 13 (adjacent to the positiveelectrode 12) and negative electrode (negative electrode active materiallayer) 15 (adjacent to the separator layer 13) in combination form asingle electric cell layer 16. With a plurality of stacked electric celllayers 16, the lithium ion secondary battery 10 of the first embodimenthas such a structure that the electric cell layers 16 are electricallyconnected in parallel. In addition, each of outermost positive electrodecurrent collectors 11 a positioned in respective outermost parts(uppermost and lowermost in FIG. 2) of the generating element (cellelement; stack structure) 17 has one face alone that is formed with thepositive electrode (positive electrode active material layer) 12.Otherwise, the structure in FIG. 2 may be so modified that each ofoutermost negative electrode current collectors (not shown in FIG. 2)positioned in respective outermost parts (uppermost and lowermost inFIG. 2) of the generating element (cell element; stack structure) 17 hasone face alone that is formed with the negative electrode (negativeelectrode active material layer) 15.

Moreover, a positive electrode tab 18 and a negative electrode tab 19conductive with the respective positive electrode plate and negativeelectrode plate are mounted to the respective positive electrode currentcollector 11 and negative electrode current collector 14 via a positiveelectrode terminal lead 20 and a negative electrode terminal lead 21respectively, where an ultrasonic welding, a resistance welding or thelike works for the above mounting operation. As such, being sandwichedby the heat sealed portions, the positive electrode tab 18 and thenegative electrode tab 19 each are exposed outward from the cell outerpackage 22.

Second Embodiment

FIG. 3 shows a typical bipolar lithium ion secondary battery, accordingto a second embodiment of the present invention. More specifically, FIG.3 shows a schematic of a cross sectional view of an entire structure ofa flat (stack) bipolar lithium ion secondary battery (hereinafter,otherwise referred to as “bipolar lithium ion secondary battery” or“bipolar secondary battery” for short).

As shown in FIG. 3, a bipolar lithium ion secondary battery 30 accordingto the second embodiment has such a structure that a substantiallyrectangular generating element (cell element) 37 for actually promotingcharging and discharging reactions is sealed and received in a cellouter package 42. As shown in FIG. 3, the generating element (cellelement) 37 of the bipolar lithium ion secondary battery 30 according tothe second embodiment has such a structure that two or more of thebipolar electrodes 34 sandwich therebetween a separator layer 35.Hereinabove, via the separator layer 35, a positive electrode (positiveelectrode active material layer) 32 of the bipolar electrode 34 opposesa negative electrode (negative electrode active material layer) 33 ofthe adjacent bipolar electrode 34. Herein, the bipolar electrode 34 hasa current collector 31 having a first face formed with the positiveelectrode (positive electrode active material layer) 32 and a secondface formed with the negative electrode (negative electrode activematerial layer) 33. In other words, the bipolar lithium ion secondarybattery 30 has such a structure that the generating element 37 includesa plurality of bipolar electrodes 34 which are stacked via the separatorlayers 35.

The positive electrode (positive electrode active material layer) 32,the separator layer 35 (adjacent to the positive electrode 32) and thenegative electrode (negative electrode active material layer) 33(adjacent to the separator layer 35) form a single electric cell layer36 (otherwise referred to as “cell unit” or “unit cell”). As such, it isalso interpreted that the bipolar lithium ion secondary battery 30 hassuch a structure that the electric cell layers 36 are stacked. Moreover,the periphery of the electric cell layer 36 has a seal portion(insulator layer) 43 for preventing a liquid junction attributable tothe electrolytic solution leaking from the separator layer 35. Providingthe seal portion (insulator layer) 43 can insulate the adjacent currentcollectors 31 from each other, and prevent a short circuit which may becaused by a contact between the positive electrode 32 and the negativeelectrode 33 adjacent to each other via the separator layer 35.

In addition, an outermost positive electrode side electrode 34 a and anoutermost negative electrode side electrode 34 b of the generatingelement (cell element) 37 may have a structure other than a bipolarelectrode. In other words, the electrodes 34 a, 34 b may have therespective positive electrode (positive electrode active material layer)32 and negative electrode (negative electrode active material layer) 33each disposed only on the first face which is necessary for one of therespective current collectors 31 a, 31 b (or terminal plates).Specifically, the positive electrode (positive electrode active materiallayer) 32 may be disposed only on the first face of the positiveelectrode side outermost current collector 31 a in the generatingelement (cell element) 37. Likewise, the negative electrode (negativeelectrode active material layer) 33 may be disposed only on the firstface of the negative electrode side outermost current collector 31 b inthe generating element (cell element) 37. Moreover, the bipolar lithiumion secondary battery 30 has such a structure that a positive electrodetab 38 and a negative electrode tab 39 are connected respectively to thepositive electrode side outermost current collector 31 a (uppermost) andthe negative electrode side outermost current collector 31 b(lowermost), when necessary, via a positive electrode terminal lead 40and a negative electrode terminal lead 41 respectively. Otherwise,extension of the positive electrode side outermost current collector 31a may serve as the positive electrode tab 38 to be lead out of the cellouter package 42 which is a laminate sheet, likewise, extension of thenegative electrode side outermost current collector 31 b may serve asthe negative electrode tab 39 to be lead out of the cell outer package42 which is a laminate sheet.

Moreover, for preventing an external shock or an environmentaldeterioration during usage, the bipolar lithium ion secondary battery 30may have such a structure that the generating element (cell element;stack structure) 37 is enclosed in the cell outer package 42 in adepressurized manner and the positive electrode tab 38 and the negativeelectrode tab 39 are taken out of the cell outer package 42. The basicstructure of the bipolar lithium ion secondary battery 30 has aplurality of stacked electric cell layers 36 (cell units or unit cells)connected in series.

As set forth above, structural elements and production methods of eachof the non-bipolar lithium ion secondary battery 10 and the bipolarlithium ion secondary battery 30 are substantially the same, except thatthe electric connection (electrode structure) in the lithium ionsecondary batteries 10, 30 are different from each other, i.e.,“connected in parallel” for the former while “connected in series” forthe latter. Moreover, the non-bipolar lithium ion secondary battery 10and bipolar lithium ion secondary battery 30 of the present inventioncan be used for pack batteries and vehicles.

Third Embodiment External Structure of Lithium Ion Secondary Battery

FIG. 4 shows a typical lithium ion secondary battery, that is, aperspective view of a flat stack non-bipolar or bipolar lithium ionsecondary battery, according to a third embodiment of the presentinvention.

As shown in FIG. 4, a flat stack lithium ion secondary battery 50 isflat and rectangular, with first and second sides thereof formedrespectively with a positive electrode tab 58 and a negative electrodetab 59 for taking out electric power. A generating element (cellelement) 57 is packed with a cell outer package 52 of the lithium ionsecondary battery 50 and has a periphery which is heat sealed. Thegenerating element 57 is sealed in such a state the positive electrodetab 58 and the negative electrode tab 59 are pulled out. Herein, thegenerating element (cell element) 57 is a counterpart of each of thegenerating element (cell element) 17 of the non-bipolar lithium ionsecondary battery 10 in FIG. 2 and the generating element (cell element)37 of the bipolar lithium ion secondary battery 30 in FIG. 3. Moreover,the generating element (cell element) 57 is a stack of the electric celllayers (electric cells) 16, 36 including the positive electrodes(positive electrode active material layers) 12, 32, separator layers 13,35 and negative electrodes (negative electrode active material layers)15, 33.

In addition, configuration of the lithium ion secondary battery of thepresent invention is not specifically limited to being stacked and flatas shown in FIG. 2 and FIG. 3, other examples thereof including rolledlithium ion secondary battery which is cylindrical. Otherwise, the abovecylinder may be modified into a rectangular flat configuration. Theabove cylindrical lithium ion secondary battery may have an outerpackage using a laminate film or a conventional cylindrical can (metalcan).

Moreover, the sides for taking out the positive and negative electrodetabs 58, 59 are not specifically limited to those shown in FIG. 4. Thepositive and negative electrode tabs 58, 59 may be taken out from thesame side. Otherwise, a plurality of positive electrode tabs 58 and aplurality of negative electrode tabs 59 may be taken out from first andsecond sides respectively. Moreover, in the case of the rolled lithiumion secondary battery, for serving as a terminal, the cylindrical can(or metal can) can replace the positive and negative electrode tabs 58,59.

As a high capacity power source for an electric car, a hybrid electriccar, a fuel cell car, and a hybrid fuel cell car and the like, thelithium ion secondary battery of the present invention can be preferablyused for a vehicle driving power source or an auxiliary power source forcausing demanded high volume energy density and high volume outputdensity.

Fourth Embodiment Pack Battery

A pack battery of the present invention has such a structure that aplurality of lithium ion secondary batteries of the present inventionare connected together, more in detail, two or more of the lithium ionsecondary batteries are connected in series, parallel or both.Connecting the lithium ion secondary batteries in series or parallel canarbitrarily adjust capacity and voltage of the batteries. Otherwise, thepack battery of the present invention may have such a structure that thenon-bipolar lithium ion secondary battery (or batteries) and bipolarlithium ion secondary battery (or batteries) of the present inventionare connected in series, parallel or both.

FIG. 5A, FIG. 5B and FIG. 5C show a typical pack battery, according to afourth embodiment of the present invention, where FIG. 5A is a plan viewof the pack battery, FIG. 5B is a front view of the pack battery andFIG. 5C is a side view of the pack battery.

As shown in FIG. 5A, FIG. 5B and FIG. 5C, a pack battery 300 of thepresent invention has such a structure that a plurality of lithium ionsecondary batteries are connected in series or parallel, thus forming asmall pack battery 250 which is attachable and detachable. Moreover, aplurality of small pack batteries 250 are connected in series orparallel, to thereby form the high-capacity and high-output pack battery300 which is proper for a vehicle driving power source or an auxiliarypower source for causing demanded high volume energy density and highvolume output density. With FIG. 5A, FIG. 5B and FIG. 5C respectivelyshowing plan view, front view and side view of the pack battery 300, thesmall pack batteries 250 (attachable-detachable) thus prepared aremutually connected by means of electric connectors such as bus bar andare stacked by means of a connecting jig 310. How many non-bipolar orbipolar lithium ion secondary batteries are connected for preparing thesmall pack battery 250 and how many small pack batteries 250 are stackedfor preparing the pack battery 300 are determined according to cellcapacity or output of the vehicle (electric car).

Fifth Embodiment Vehicle

A vehicle of the present invention includes the lithium ion secondarybattery of the present invention or the pack battery which is acombination of a plurality of above lithium ion secondary batteries.Using the high capacity positive electrode of the present invention canmake a battery causing a high energy density, thus accomplishing aplug-in hybrid car featuring a long EV (=Electric Vehicle) traveldistance and an electric car featuring a long travel distance percharge. In other words, under the present invention, the lithium ionsecondary battery or the pack battery which is a combination of aplurality of lithium ion secondary batteries can be used for a drivingsource of the vehicle. Examples of the vehicle having a long life and ahigh reliability include four wheelers such as hybrid car, fuel cell carand electric car; two wheelers (motor bike); three wheelers and thelike, where the four wheelers including passenger car, truck, commercialvehicle such as bus, mini vehicle and the like. The application of thelithium ion secondary battery or pack battery of the present inventionis not specifically limited to car, other examples including variouspower sources of mobile bodies such as electric trains, installed powersources such as non-outage power source, and the like.

FIG. 6 is a schematic of a car including the pack battery, according toa fifth embodiment of the present invention.

As shown in FIG. 6, the pack battery 300 is installed below a seat of acentral part of a body of an electric car 400. Installing below the seatcan keep a spacious passenger space and a wide trunk. The position forinstalling the pack battery 300 is, however, not limited to below theseat, other examples thereof including below a rear trunk, in a frontengine room and the like. The electric car 400 including the above packbattery 300 has a high durability and keeps a sufficient output for along-time operation. In addition, the above pack battery 300 works forproviding an electric car, a hybrid car and the like which are excellentin fuel economy and traveling performance. The above pack battery 300 isalso applicable to a hybrid car, a fuel cell car and the like, otherthan the electric vehicle 400 in FIG. 6.

EXAMPLES

Hereinafter, effects brought about by the present invention are to beset forth referring to the following examples and comparative examples.The scope of the present invention is, however, not limited to theexamples.

Example 1 Preparation of Positive Electrode Layer

LiMn₂O₄ (average particle diameter: 10 μm) (90 mass part) as a positiveelectrode active material, carbon black (6 mass part) as a conductiveassistant, and polyvinylidene fluoride (PVDF #1300) (4 mass part) as abinder were mixed. The thus prepared mixture as a positive electrodemixture was dispersed with N-methyl-2-pyrrolidone (50 mass part) as asolvent, to thereby obtain a slurry. The thus prepared slurry wasapplied to an aluminum (Al) foil (serving as a current collector andhaving a thickness of 20 μm), followed by pressing and drying, so as tofinally prepare a positive electrode layer having a thickness of 70 μm.As such, the positive electrode layer was prepared.

[Preparation of Negative Electrode Layer]

An artificial graphite powder (average particle diameter: 10 μm) (90mass part) as a negative electrode active material and polyvinylidenefluoride (PVDF #9200) (10 mass part) as a binder were dispersed withN-methyl-2-pyrrolidone (50 mass part) as a solvent, to thereby obtain aslurry. The thus obtained slurry was applied to a copper (Cu) foil(serving as a negative electrode current collector and having athickness of 20 μm), followed by drying and pressing, so as to finallyprepare a negative electrode layer having a thickness of 40 μm. As such,the negative electrode layer was prepared.

[Preparation of Electrolytic Solution]

Ethylene carbonate (30 volume part) and diethyl carbonate (70 volumepart) were mixed as a solvent. Then, by a ratio of 1 mol/dm³, LiPF₆ as asolute was added to the mixed solvent, to thereby prepare a non-aqueouselectrolytic solution.

[Preparation of Solid Electrolytic Precursor Solution]

Polyethylene oxide as polymer electrolyte (40 mass %) was mixed with thenon-aqueous electrolytic solution (60 mass %) prepared above.Azobisisobutyronitrile (AIBN) as a thermal polymerizing initiatorequivalent to 5000 mass ppm relative to the polymer electrolyte wasadded to the above mixture, to thereby prepare a solid electrolyticprecursor solution.

Polyolefin film {made of polyethylene (PE), thickness of 10 μm, voidageof 45%, curvature of 1.5} as a separator substrate was dipped in avessel filled with the solid electrolytic precursor solution obtainedabove, followed by a vacuum impregnation at an ambient temperature for 1hr. Then, the polyethylene film was sandwiched by parting films, thenwas lightly brandished with a roll for removing an excessive solidelectrolytic precursor solution, to thereby obtain an impregnatedseparator. Then, the impregnated separator was sandwiched between theabove prepared positive electrode layer and negative electrode layer,was put into a laminate bag, was sandwiched (for fixation) between twoglass plates on both sides, followed by a thermal polymerizing in anoven at 80° C. for 3 hr, to thereby obtain a stack structure of thepositive electrode layer, separator layer and negative electrode layer.As a result, the polymerizing of the solid electrolytic precursor worksfor forming the contact faces (between positive electrode layer andseparator layer and between negative electrode layer and separatorlayer) in a state of adhesion. In this case, each of the positiveelectrode layer and negative electrode layer is free from entry ofelectrolytic solution.

Then, the electrolytic solution was injected to the stack structure(including the positive electrode layer, separator layer and negativeelectrode layer) sandwiched between the two glass plates, and then thestack structure was packed with a laminate in a vacuum state, to therebyprepare a stack secondary battery.

The thus prepared stack secondary battery has such a structure that theelectrolyte of each of the negative electrode layer and the positiveelectrode layer is a liquid material while the electrolyte of theseparator layer is a polymer. Moreover, in the stack secondary battery,each of the negative electrode layer and the positive electrode layeruses a liquid electrolyte having a conductivity of 2×10⁻³ (S/cm) whilethe separator layer uses a gel polymer electrolyte having a conductivityof 6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the positive electrode layer or negative electrode layer isinterpenetrated (held) in the separator layer.

According to the following methods, charging and dischargingcharacteristic tests were implemented on the thus prepared secondarybattery. Table 1 shows results of the tests.

<Charging and Discharging Characteristic Test>

The charging and discharging characteristic test has the followingconditions:

1: Constant current charging (charging at a certain charging current tosuch an extent as to obtain a certain charging voltage, then, keeping atotal charging period of 15 hr at the certain charging voltage).

2: Rest (10 min).

3: Constant voltage discharging (discharging at a certain dischargingcurrent to such an extent as to obtain a certain discharging voltage).

4: Rest (10 min).

Hereinabove, certain charging current: 0.1 C, certain charging voltage:4.2 V, and certain discharging voltage: 2 V.

1 C is defined as a current for fully (100%) charging a battery in 1 hr.

For example, 2 C is two times 1 C and capable of fully charging thebattery in 30 min.

Moreover, the certain discharging current was adjusted to be 0.2 C forthe first cycle, 0.5 C for the second cycle, and 0.2 C for the thirdcycle and thereafter.

Discharging efficiency was defined as a ratio of discharging capacitywhen the polymer electrolyte is used relative to discharging capacitywhen the liquid electrolyte alone is used as electrolyte at 0.1 C forcharging and 0.2 C for discharging. In other words, the dischargingefficiency is given by the following expression: Discharging efficiency(%)=(discharging capacity when polymer electrolyte is used/dischargingcapacity when liquid electrolyte alone is used as electrolyte)×100

Example 2

The method of the example 1 was repeated in the example 2, to therebyprepare the positive electrode layer. The thus prepared positiveelectrode layer was dipped in a vessel filled with a solid electrolyticprecursor solution prepared like that according to the example 1,followed by a vacuum impregnation at an ambient temperature for 1 hr.Then, the positive electrode layer was sandwiched by parting films thenwas lightly brandished with a roll for removing an excessive solidelectrolytic precursor solution, to thereby obtain an impregnatedpositive electrode layer.

Except that the thus prepared impregnated positive electrode layer wasused as a positive electrode layer, the example 1 was repeated, i.e.,the thermal polymerizing was implemented, to thereby obtain a stackstructure of the positive electrode layer, separator layer and negativeelectrode layer. As a result, the polymerizing of the solid electrolyticprecursor works for forming the contact faces (between positiveelectrode layer and separator layer and between negative electrode layerand separator layer) in a state of adhesion. In this case, the negativeelectrode layer is free from entry of electrolytic solution.

Then, the method of the example 1 was likewise repeated, to therebyprepare a stack secondary battery.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the negative electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the positiveelectrode layer is a polymer. Moreover, in the stack secondary battery,the negative electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thepositive electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm). As such, the gel polymer electrolytehaving a conductivity lower than that of the liquid electrolyteinterpenetrated (held) in the negative electrode layer isinterpenetrated (held) in the separator layer and positive electrodelayer.

According to the following methods, charging and dischargingcharacteristic tests were implemented on the thus prepared secondarybattery. Table 1 shows results of the tests.

Example 3 Preparation of Positive Electrode Layer

LiMn₂O₄ (average particle diameter: 10 μm) (90 mass part) as a positiveelectrode active material, carbon black (6 mass part) as a conductiveassistant, and polyvinylidene fluoride (PVDF #1300) (4 mass part) as abinder were mixed. The thus prepared mixture as a positive electrodemixture was dispersed with N-methyl-2-pyrrolidone (50 mass part) as asolvent, to thereby obtain a slurry. The thus prepared slurry wasapplied to an aluminum (Al) foil (serving as a current collector andhaving a thickness of 20 μm), followed by drying and pressing, so as tofinally prepare a positive electrode layer having a thickness of 36 μm.As such, a first positive electrode layer was prepared. In this case,the first positive electrode layer had a voidage of 35%. Then, theslurry like that set forth above was applied to the first positiveelectrode layer, followed by pressing (at a pressure lower than that forpreparing the first positive electrode layer) and drying, so as tofinally prepare a positive electrode layer having a thickness of 40 μm.As such, a second positive electrode layer was prepared. In this case,the second positive electrode layer had a voidage of 40%.

[Preparation of Negative Electrode Layer]

Artificial graphite powder (average particle diameter: 10 μm) (90 masspart) as a negative electrode active material and polyvinylidenefluoride (PVDF #9200) (10 mass part) as a binder were mixed weredispersed with N-methyl-2-pyrrolidone (50 mass part) as a solvent, tothereby obtain a slurry. The thus obtained slurry was applied to acopper (Cu) foil (serving as a negative electrode current collector andhaving a thickness of 20 μm), followed by pressing and drying, so as tofinally prepare a negative electrode layer having a thickness of 20 μm.As such, a first negative electrode layer was prepared. In this case,the first negative electrode layer had a voidage of 35%. Then, theslurry like that set forth above was applied to the first negativeelectrode layer, followed by pressing (at a pressure lower than that forpreparing the first negative electrode layer) and drying, so as tofinally prepare a negative electrode layer having a thickness of 25 μm.As such, a second negative electrode layer was prepared. In this case,the second negative electrode layer had a voidage of 40%.

Except that the thus prepared positive electrode layer and negativeelectrode layer were used, operations like those of the example 1 wereimplemented, to thereby obtain a stack secondary battery.

The thus prepared stack secondary battery has such a structure that theelectrolyte of each of the negative electrode layer and positiveelectrode layer is a liquid material while the electrolyte of theseparator layer is a polymer. Moreover, in the stack secondary battery,each of the negative electrode layer and positive electrode layer uses aliquid electrolyte having a conductivity of 2×10⁻³ (S/cm) while theseparator layer uses a gel polymer electrolyte having a conductivity of6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the positive electrode layer or negative electrode layer isinterpenetrated (held) in the separator layer.

According to the following methods, charging and dischargingcharacteristic tests were implemented on the thus prepared secondarybattery. Table 1 shows results of the tests.

Example 4

Except that the positive electrode layer and negative electrode layerused for the example 3 were used for the example 2, operations likethose of the example 2 were implemented in the example 4, to therebyprepare a stack secondary battery.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the negative electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the positiveelectrode layer is a polymer. Moreover, in the stack secondary battery,the negative electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thepositive electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm). As such, the gel polymer electrolytehaving a conductivity lower than that of the liquid electrolyteinterpenetrated (held) in the negative electrode layer isinterpenetrated (held) in the separator layer and positive electrodelayer.

According to the following methods, charging and dischargingcharacteristic tests were implemented on the thus prepared secondarybattery. Table 1 shows results of the tests.

Example 5

The example 1 was repeated except that the following 1) and 2) wereimplemented, to thereby prepare a stack secondary battery according tothe example 5:

1) The positive electrode layer was prepared by using an aluminum (Al)foil (thickness: 20 μm) having a surface formed with grooves forinterpenetrating electrolytic solution, where the grooves have intervalsof 1 μm therebetween and have a volumetric ratio of 2.5% relative to acurrent collector, each of the grooves having a width of 1 μm and adepth of 1 μm.

2) Moreover, the negative electrode layer was prepared by using a copper(Cu) foil (thickness: 20 μm) having a surface formed with grooves forinterpenetrating electrolytic solution, where the grooves have intervalsof 1 μm therebetween and have a volumetric ratio of 2.5% relative to acurrent collector, each of the grooves having a width of 1 μm and adepth of 1 μm.

Moreover, according to the example 5, a stack structure of the positiveelectrode layer, separator layer and negative electrode layer was put ina laminate bag such that the grooves in each of the aluminum (Al) foiland the copper (Cu) foil are arranged in parallel to a direction ofinjecting the electrolytic solution.

The thus obtained stack secondary battery has such a structure that theelectrolyte of each of the negative electrode layer and positiveelectrode layer is a liquid material while the electrolyte of theseparator layer is a polymer. Moreover, in the stack secondary battery,each of the negative electrode layer and positive electrode layer uses aliquid electrolyte having a conductivity of 2×10⁻³ (S/cm) while theseparator layer uses a gel polymer electrolyte having a conductivity of6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the positive electrode layer or negative electrode layer isinterpenetrated (held) in the separator layer.

According to the following methods, charging and dischargingcharacteristic tests were implemented on the thus prepared secondarybattery. Table 1 shows results of the tests.

Example 6

The example 2 was repeated except that the following 1) and 2) wereimplemented, to thereby prepare a stack secondary battery according tothe example 6:

1) The positive electrode layer was prepared by using an aluminum (Al)foil (thickness: 20 μm) having a surface formed with grooves forinterpenetrating electrolytic solution, where the grooves have intervalsof 1 μm therebetween and have a volumetric ratio of 2.5% relative to acurrent collector, each of the grooves having a width of 1 μm and adepth of 1 μm.

2) Moreover, the negative electrode layer was prepared by using a copper(Cu) foil (thickness: 20 μm) having a surface formed with grooves forinterpenetrating electrolytic solution, where the grooves have intervalsof 1 μm therebetween and have a volumetric ratio of 2.5% relative to acurrent collector, each of the grooves having a width of 1 μm and adepth of 1 μm.

The thus obtained stack secondary battery has such a structure that theelectrolyte of the negative electrode layer is a liquid material whilethe electrolyte of each of the separator layer and positive electrodelayer is a polymer. Moreover, in the stack secondary battery, thenegative electrode layer uses a liquid electrolyte having a conductivityof 2×10⁻³ (S/cm) while each of the separator layer and positiveelectrode layer uses a gel polymer electrolyte having a conductivity of6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the negative electrode layer is interpenetrated (held) in theseparator layer and positive electrode layer.

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Example 7 Preparation of Positive Electrode Layer

LiMn₂O₄ (average particle diameter: 10 μm) (90 mass part) as a positiveelectrode active material, carbon black (6 mass part) as a conductiveassistant, and polyvinylidene fluoride (PVDF #1300) (4 mass part) as abinder were mixed. The thus prepared mixture as a positive electrodemixture was dispersed with N-methyl-2-pyrrolidone (50 mass part) as asolvent, to thereby obtain a slurry. The thus prepared slurry wasapplied (patterned) to an aluminum (Al) foil (serving as a currentcollector and having a thickness of 20 μm), so as to prepare grooves forinterpenetrating electrolytic solution, which is a preparation of afirst active material sublayer. In this case, the grooves have avolumetric ratio of 6.25% relative to a resultant positive electrodelayer, each of the grooves having a width of 10 μm and a depth of 10 μm.Then, a second active material sublayer was applied (transcribed) to thethus patterned first active material sublayer for forming the grooves,followed by pressing and drying, so as to finally prepare the resultantpositive electrode layer having a thickness of 80 μm. As such, theresultant positive electrode layer including the first and second activematerial sublayers and the current collector was prepared.

[Preparation of Negative Electrode Layer]

Artificial graphite powder (average particle diameter: 10 μm) (90 masspart) as a negative electrode active material and polyvinylidenefluoride (PVDF #9200) (10 mass part) as a binder were mixed weredispersed with N-methyl-2-pyrrolidone (50 mass part) as a solvent, tothereby obtain a slurry. The thus prepared slurry was applied(patterned) to a copper (Cu) foil (serving as a negative electrodecurrent collector and having a thickness of 20 μm), so as to preparegrooves for interpenetrating electrolytic solution, which is apreparation of a first active material sublayer. In this case, thegrooves have a volumetric ratio of 10% relative to a resultant negativeelectrode layer, each of the grooves having a width of 10 μm and a depthof 10 μm. Then, a second active material sublayer was applied(transcribed) to the thus patterned first active material sublayer forforming the grooves, followed by pressing and drying, so as to finallyprepare the resultant negative electrode layer having a thickness of 50μm. As such, the resultant negative electrode layer including the firstand second active material sublayers and the current collector wasprepared. Moreover, according to the example 7, a stack structure of thepositive electrode layer, separator layer and negative electrode layerwas put in a laminate bag such that the grooves in each of the positiveelectrode layer and negative electrode layer are arranged in parallel toa direction of injecting the electrolytic solution.

Except that the thus prepared positive electrode layer and negativeelectrode layer were used, the stack secondary battery according to theexample 7 was prepared in a manner like that of the example 1.

The thus prepared stack secondary battery has such a structure that theelectrolyte of each of the negative electrode layer and positiveelectrode layer is a liquid material while the electrolyte of theseparator layer is a polymer. Moreover, in the stack secondary battery,each of the negative electrode layer and positive electrode layer uses aliquid electrolyte having a conductivity of 2×10⁻³ (S/cm) while theseparator layer uses a gel polymer electrolyte having a conductivity of6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the positive electrode layer or negative electrode layer isinterpenetrated (held) in the separator layer.

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Example 8

Except that the positive electrode layer and negative electrode layerprepared in the example 7 were used in the example 2, the stacksecondary battery according to the example 8 was prepared in a mannerlike that of the example 2.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the negative electrode layer is a liquid material whilethe electrolyte of each of the separator layer and positive electrodelayer is a polymer. Moreover, in the stack secondary battery, thenegative electrode layer uses a liquid electrolyte having a conductivityof 2×10⁻³ (S/cm) while each of the separator layer and positiveelectrode layer uses a gel polymer electrolyte having a conductivity of6×10⁻⁴ (S/cm). As such, the gel polymer electrolyte having aconductivity lower than that of the liquid electrolyte interpenetrated(held) in the negative electrode layer is interpenetrated (held) in theseparator layer and positive electrode layer.

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Comparative Example 1

According to the comparative example 1, the positive electrode layer andnegative electrode layer were dipped in a vessel to be subjected to avacuum impregnation at an ambient temperature for 1 hr, like thepolyolefin film as the separator substrate in the example 1. Then, thenegative electrode layer overlapped with the polyethylene film wassandwiched by parting films, then the thus obtained was lightlybrandished with a roll for removing an excessive solid electrolyticprecursor solution. The thus obtained was overlapped with a driednegative electrode layer, was put into a laminate bag and was sandwichedbetween glass plates on both sides for pressurizing, followed by athermal polymerizing in an oven at 80° C. for 3 hr. In the above state,the separator layer has a contact with each of the positive electrodelayer and negative electrode layer. Then, the secondary battery wasprepared according to the comparative example 1, like according to theexample 1. A gel polymer electrolyte having a conductivity of 6×10⁻⁴(S/cm) was used for the positive electrode layer, separator layer andnegative electrode layer.

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Comparative Example 2

The example 1 was repeated to thereby prepare the negative electrodelayer. The thus prepared negative electrode layer was dipped in a vesselfilled with the solid electrolytic precursor solution prepared in amanner like that of the example 1, followed by a vacuum impregnation atan ambient temperature for 1 hr. Then, the negative electrode layer wassandwiched by parting films then was lightly brandished with a roll forremoving an excessive solid electrolytic precursor solution, to therebyobtain an impregnated negative electrode layer.

Except that the thus impregnated negative electrode layer was used as anegative electrode layer, the example 1 was repeated, i.e., the thermalpolymerizing was implemented, to thereby obtain a stack structure of thepositive electrode layer, separator layer and negative electrode layer.The above positive electrode layer is free of entry of electrolyticsolution.

Then, the method of the example 1 was likewise repeated, to therebyprepare a stack secondary battery.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the positive electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the negativeelectrode layer is a polymer. Moreover, in the stack secondary battery,the positive electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thenegative electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm).

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Comparative Example 3

Except that the positive electrode layer and negative electrode layerused in the example 3 were used in the comparative example 2, the stacksecondary battery according to the comparative example 3 was prepared ina manner like that of the comparative example 2.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the positive electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the negativeelectrode layer is a polymer. Moreover, in the stack secondary battery,the positive electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thenegative electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm).

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Comparative Example 4

Except that the positive electrode layer and negative electrode layerused in the example 5 were used in the comparative example 2, the stacksecondary battery according to the comparative example 4 was prepared ina manner like that of the comparative example 2.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the positive electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the negativeelectrode layer is a polymer. Moreover, in the stack secondary battery,the positive electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thenegative electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm).

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

Comparative Example 5

Except that the positive electrode layer and negative electrode layerprepared in the example 7 were used in the comparative example 2, thestack secondary battery according to the comparative example 5 wasprepared in a manner like that of the comparative example 2.

The thus prepared stack secondary battery has such a structure that theelectrolyte of the positive electrode layer is a liquid material whilethe electrolyte of each of the separator layer and the negativeelectrode layer is a polymer. Moreover, in the stack secondary battery,the positive electrode layer uses a liquid electrolyte having aconductivity of 2×10⁻³ (S/cm) while each of the separator layer and thenegative electrode layer uses a gel polymer electrolyte having aconductivity of 6×10⁻⁴ (S/cm).

Charging and discharging characteristic tests were implemented on thethus prepared secondary battery according to the following methods.Table 1 shows results of the tests.

TABLE 1 Discharging efficiency (%) 0.2 C 0.5 C Example 1 90% 85% Example2 81% 56% Example 3 95% 91% Example 4 85% 68% Example 5 95% 90% Example6 84% 63% Example 7 95% 92% Example 8 83% 65% Comparative example 1 62%38% Comparative example 2 65% 41% Comparative example 3 66% 43%Comparative example 4 67% 41% Comparative example 5 68% 45%

In view of the examples 1 and 2 compared with the comparative examples 1and 2; the examples 3 and 4 compared with the comparative example 3; theexamples 5 and 6 compared with the comparative example 4; and theexamples 7 and 8 compared with the comparative example 5, it is foundthat the batteries of the present invention are more excellent indischarging efficiency have higher capacity and cause higher output.

Although the present invention has been described above by reference tocertain embodiments and examples, the present invention is not limitedto the embodiment and examples described above. Modifications andvariations of the embodiments and examples described above will occur tothose skilled in the art, in light of the above teachings.

This application is based on a prior Japanese Patent Application Nos.P2007-150802 (filed on Jun. 6, 2007 in Japan) and P2008-031801 (filed onFeb. 13, 2008 in Japan). The entire contents of the Japanese PatentApplication Nos. P2007-150802 and P2008-031801 from which priorities areclaimed are incorporated herein by reference, in order to take someprotection against translation errors or omitted portions.

The scope of the present invention is defined with reference to thefollowing claims.

What is claimed is:
 1. A method of producing a secondary battery, themethod comprising the following sequential operations: dipping aseparator in a polymer electrolytic precursor solution to prepare animpregnated separator; adhering the impregnated separator to each of apositive electrode layer and a negative electrode layer by sandwichingthe impregnated separator between the positive and negative electrodelayers and polymerizing an electrolyte of the impregnated separatorwhile applying a pressure to the positive electrode layer, theimpregnated separator and the negative electrode layer in a direction ofthickness of the positive electrode layer, the impregnated separator andthe negative electrode layer, to thereby form a stack structure of thepositive electrode layer, a separator layer and the negative electrodelayer; and injecting a liquid electrolyte to the stack structure,wherein a conductivity of an electrolyte of the negative electrode layerin the secondary battery is higher than a conductivity of an electrolyteof at least one of the separator layer and the positive electrode layer.2. The method of producing the secondary battery according to claim 1,wherein in the injection operation, a pressure is applied to a face ofthe stack structure from a direction substantially perpendicular to theface of the stack structure such that a thickness of the stack structurehas a variation in a range within 5% of a certain thickness.
 3. Themethod of producing the secondary battery according to claim 1, furthercomprising: at least one of the following first and second operations: afirst operation of forming the positive electrode layer, including:forming a positive electrode current collector having a groove defininga width and a depth, and forming a positive electrode active materiallayer on the positive electrode current collector, wherein each of thewidth and the depth of the groove of the positive electrode currentcollector is less than or equal to 10% of an average particle diameterof the positive electrode active material layer, and a second operationof forming the negative electrode layer, including: forming a negativeelectrode current collector having a groove defining a width and adepth, and forming a negative electrode active material layer on thenegative electrode current collector, wherein each of the width and thedepth of the groove of the negative electrode current collector is lessthan or equal to 10% of an average particle diameter of the negativeelectrode active material layer.
 4. A method of producing a secondarybattery, the method comprising the following sequential operations:dipping a separator in a polymer electrolytic precursor solution toprepare an impregnated separator; adhering the impregnated separator toeach of a positive electrode layer and a negative electrode layer bysandwiching the impregnated separator between the positive and negativeelectrode layers and polymerizing an electrolyte of the impregnatedseparator while applying a pressure to the positive electrode layer, theimpregnated separator and the negative electrode layer in a direction ofthickness of the positive electrode layer, the impregnated separator andthe negative electrode layer, to thereby form a stack structure of thepositive electrode layer, a separator layer and the negative electrodelayer; injecting a liquid electrolyte to the stack structure; andvacuum-impregnating the stack structure, wherein a conductivity of anelectrolyte of the negative electrode layer in the secondary battery ishigher than a conductivity of an electrolyte of at least one of theseparator layer and the positive electrode layer.
 5. The method ofproducing the secondary battery according to claim 4, wherein in theinjecting operation, a pressure is applied to a face of the stackstructure from a direction substantially perpendicular to the face ofthe stack structure, such that a thickness of the stack structure has avariation in a range within 5% of a certain thickness.
 6. The method ofproducing the secondary battery according to claim 4, furthercomprising: at least one of the following first and second operations: afirst operation of forming the positive electrode layer, including:forming a positive electrode current collector having a groove defininga width and a depth, and forming a positive electrode active materiallayer on the positive electrode current collector, wherein each of thewidth and the depth of the groove of the positive electrode currentcollector, is less than or equal to 10% of an average particle diameterof the positive electrode active material layer, and a second operationof forming the negative electrode layer, including: forming a negativeelectrode current collector, having a groove defining a width and adepth, and forming a negative electrode active material layer on thenegative electrode current collector, wherein each of the width and thedepth of the groove of the negative electrode current collector is lessthan or equal to 10% of an average particle diameter of the negativeelectrode active material layer.